Coal-fired utility boilers produce NOx and SO.sub.2 emissions which cause acid rain. Extensive studies have shown that the NOx-related portion of acid rain components can produce extensive damage to trees and forests, and NOx control technology for coal-fired boilers may become required in many industrialized nations, in addition to the NOx controls presently required in Japan and West Germany. NOx control is also an important consideration for incineration systems, where there is currently no demonstrated NOx control technology for achieving 70% or more NOx removal.
Many existing coal-fired boilers or solid waste incinerators already have wet scrubbers for controlling SO.sub.2 or HCl emissions. Specifically, wet scrubbers have been installed on about 60,000 MWe of coal-fired generating capacity in the U.S. and on about 40,000 MWe in West Germany Many other countries are presently requiring the installation of wet scrubbers for coal-fired boilers and solid waste incinerators. The present invention provides a means for achieving very high levels of NOx removal, on the order of 70% or more, especially in situations where coal-fired boilers or incinerators already have wet scrubbers. This invention can provide such NOx removal performance levels without the use of expensive catalytic NOx reduction techniques.
Presently-commercialized technology for NOx control consists primarily of Selective Catalytic Reduction (SCR), where ammonia gas is injected into the flue gas and reacted with NOx over a catalyst at temperatures of about 700.degree. F. to produce nitrogen gas and water vapor byproducts Typical NOx reduction levels are 80%. The catalyst bed is typically sized sufficiently large to reduce the ammonia slippage to avoid contamination of the flyash with ammonium salt deposits. As used herein, "ammonia slippage" or "ammonia slip" means the concentration of ammonia gas contained in the flue gas exit from the NOx control process. The reported cost of SCR technology varies between about $60/kw to $120/kw, depending on site conditions. The operating cost of SCR technology includes the high cost of catalyst replacement about once every two years. SCR technology is not considered applicable for incinerators due to the contamination and poisoning of the catalyst. The purpose of the present invention is to provide similar high levels of NOx removal performance without requiring the use of SCR technology, at an extremely significant savings in both capital and operating cost.
There are other NOx control processes using NHi precursors such as gaseous ammonia or liquid-phase urea, which are injected into the flue gas at temperatures above 1400.degree. F. to reduce NO to nitrogen. These processes are called Selective Non-Catalytic Reduction (SNCR), and suffer the disadvantage that if sufficient NHi precursor material is injected to achieve high NOx removal efficiency, then there may be an unacceptably high degree of ammonia slippage. The ammonia slip combines with SO.sub.2, SO.sub.3, HCl and HF to form ammonium salts at temperatures typically less than 500.degree. F. When such salts condense, solid particulate is formed which may cause deposits in critical zones such as the air preheater system in conventional boilers. In order to prevent this problem, less of the NHi precursor material is injected, and the overall NOx reduction capability of SNCR systems is generally limited to between 30% and 60%. This level of performance does not compete with SCR systems, and is unacceptably low.
Typical SNCR processes are taught by R. K. Lyon, U.S. Pat. No. 3,900,554, and A. M. Dean et. al., U.S. Pat. No. 4,624,840, (Exxon Thermal DeNOx Process using gaseous ammonia) and by J. K. Arand et. al., U.S. Pat. No. 4,208,386 and U.S. Pat. No. 4,325,924 (EPRI/Fuel Tech NOxOUT Process using liquid-phase urea). U.S. Pat. Nos. 3,900,554, 4,624,840, 4,208,386 and 4,325,924 are incorporated herein by reference Both the Exxon and Fuel Tech processes operate preferably only within a narrow temperature window, typically between 1700.degree. and 1900.degree. F., but can operate at somewhat lower temperatures by addition of hydrogen or hydrocarbon materials to the flue gas. Typically, hydrogen is added in the Exxon process and methanol is added in the Fuel Tech process. The actual NO reduction mechanism is believed to involve a large number of radical reactions, the most important being: EQU NH.sub.2 +NO=N.sub.2 +H.sub.2 O
By use of hydrogen or hydrocarbon addition, the normal temperature window (i.e., 1800.degree. F..+-.100.degree. F.) can be lowered, for example, to 1500.degree. F..+-.100.degree. F. This involves sufficient hydrocarbon addition to increase the concentration of hydroxyl (OH) radicals, thereby increasing the concentration of NH.sub.2 radicals at lower temperatures and decreasing the ammonia slippage by the reaction: EQU NH.sub.3 +OH=NH.sub.2 +H.sub.2 O
However, this technique of adjusting the optimum temperature for an SNCR process to lower levels by addition of hydrogen, hydrogen peroxide, or hydrocarbons becomes limited below about 1500.degree. F. Below 1400.degree. F., the NO reduction efficiency of SNCR processes decreases dramatically, until below about 1300.degree. F. there is practically no reduction whatsoever.
As used herein, the term "temperature window" means the range of temperatures over which SNCR processes are effective in reducing NO to nitrogen. Typically, at very high temperatures, there is not any NO reduction. As the temperature is decreased, NO reduction performance increases until, at an optimum temperature, the NO reduction performance is maximized. Further reductions in flue gas temperature cause both rapidly increased ammonia slippage and decreased NO reduction performance, until the lower-temperature edge of the temperature window is reached, below which point there is no longer any NO reduction. As used herein, the term "optimum temperature" means the flue gas temperature for which the NO reduction performance of an SNCR process is maximized The term "SNCR process" means the use of simple or complex NHi precursors which are injected into flue gas at temperatures within the temperature window, resulting in the selective, non-catalytic reduction of NO to nitrogen. The term "simple NHi precursors" means ammonia, or other compounds, such as ammonium hydroxide or ammonium carbonate, or mixtures thereof, which liberate ammonia (NH.sub.3) upon thermal decomposition. The term "complex NHi precursors" means urea (NH.sub.2).sub. 2 CO, cyanuric acid, biuret, triuret, ammelide, other amides, or mixtures thereof, which initially liberate NH.sub.2 radicals upon thermal decomposition.
Another type of flue gas NOx control process which oxidizes NO to NO.sub.2 is disclosed in U.S. Pat. application titled: PROCESS AND APPARATUS FOR REMOVING OXIDES OF NITROGEN AND SULFUR FROM COMBUSTION GASES, Ser. No. 734,393, filed May 14, 1985 by me, now U.S. Pat. No. 4,783,325. The process disclosed in the U.S. Pat. application Ser. No. 734,393, is herein referred to as the "Jones process." U.S. Pat. No. 4,783,325 is incorporated herein by this reference. The Jones oxidation process operates between 800.degree. F. to 1400.degree. F., and utilizes a hydrocarbon material (a peroxyl initiator), such as methanol, dispersed in an air carrier to cause high levels of NO oxidation to NO.sub.2. The preferred temperature window for this process is from 800.degree. F. to 1400.degree. F. This process is a different type of boiler injection process involving the peroxyl radical (HO.sub.2) instead of hydroxyl radical (OH). NO is oxidized to NO.sub.2 by the reaction ps EQU NO+HO.sub.2 =NO.sub.2 +OH
It is seen that some appropriate temperature, say 1400.degree. F., acts as a dividing line between the NO reduction processes of the SNCR type (i.e., at 1400.degree. F. and higher) and the NO oxidation processes of the Jones type (i.e., at 1400.degree. F. and lower).